Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and diode sensors have been used for radiation dosimetry in radiation therapy [1] and other applications such as space and individual dosimetry. A MOSFET operates by trapping a positive charge in the gate oxide proportional to the absorbed dose deposited in a gate. The accumulating charge changes the threshold voltage of the MOSFET sensor measured under constant current [2]. The advantage of MOSFET sensors is in their thin sensitive volume-gate oxide (typically of less than 1 micron); this allows the measurement of dose patterns with high spatial resolution, which is important in Intensity Modulated Radiation Therapy (IMRT) and conformal therapy and brachytherapy. Another advantage of MOSFET detectors is their ability to accumulate dose during irradiation and retain dose information after multiple readouts, which is not possible with Thermo-Luminescent Dosimeter (TLD) detectors.
Silicon diodes for radiation dosimetry usually operate in a passive mode. The internal electric field produced by the p-n junction of the diode collects charge induced by radiation within the diffusion length in a base of the diode; this provides a short circuit current proportional to the radiation dose rate. Integration of this current provides a measure of the total dose. Usually the sensitive region of the diode is of the order of 50 microns. The simplicity and robustness of these diodes make them attractive for radiotherapy and in vivo real time dosimetry. In many applications diodes are used for entrance and exit dose measurements by placing a diode on a patient during irradiation. Both of these doses are associated with measurements at the water equivalent depth (WED) Dmax where charged particle equilibrium is achieved; this depth can be varied from 1.5 to 4 cm by changing the X-ray photon energy of 6 to 18 MV. For miniaturization of the diode sensor's build-up, different materials are used, such as Al or Cu for 1 to 4 MV photons, brass for 4 to 12 MV photons and Ti for 12 to 25 MV photons [3].
Various MOSFET dosimetry systems have been developed over the last 10 years. Important MOSFET parameters for radiation therapy dosimetry include the anisotropy of the response, insensitivity of the response to changes in ambient temperature and the ability to measure skin dose and depth dose. Dual MOSFET sensors have been proposed, to compensate for the temperature dependence of the threshold voltage; the gate voltages of an irradiated MOSFET sensor and a control MOSFET sensor with the same temperature coefficient are compared using a comparative circuit [4].
Another dosimetry probe comprises dual MOSFETs produced on a single substrate (so that they are essentially identical). During irradiation, the gates of the paired MOSFETs are biased with different positive voltages leading to different responses; a differential signal in test mode is then proportional to absorbed dose, while compensating for the temperature instability of the MOSFET [5].
In many radiation MOSFET probes including dual MOSFET probes, the MOSFET die—which is usually of the order of 1×1 mm and 0.35 to 0.5 mm thick—is placed on one end of a tail of Kapton brand polymer that has embedded copper leads whose ends act as connecting pads to which the substrate, source drain and gate of each MOSFET are wire bonded (with aluminium or gold wires). The other ends of the copper leads are connected to the socket plugged into the data reader. An epoxy envelope is applied to fix the bonding wires and to protect the MOSFET die from environmental conditions. The copper leads and epoxy result in additional response anisotropy especially with lower energy photons such as those used in HDR brachytherapy (where an Ir-192 source is used with an average photon energy of 360 keV) and in diagnostic techniques. This packaging has the additional problem that the shape of the epoxy is not reproducible, which results in a different build-up for each probe and differences in WED of about 0.7 to 1 mm [14].
MOSFET sensors for radiation therapy applications, whether passive or active, are presently provided as either disposable (“OneDose” [7]), multiuse (Thompson Nelson MOSFET system) or wirelessly powered and implantable MOSFETs for internal use; all have an epoxy envelope or other cover build up. This is less problematic for internal use when dose measurements are made inside the body (with an implantable wireless MOSFET [8]), where charged particle equilibrium exists and MOSFET response is driven mostly by high energy secondary electrons generated from surrounding tissue rather then from the epoxy. However, the epoxy and the current packaging makes it impossible to realize the full advantages of the micron thickness of the gate oxide of MOSFET dosimeters when used for skin dosimetry or inside anatomical cavities where dosimetry at the tissue-air interface is critical for Dose Planning System (DPS) verification. Comparable problems apply to diode sensors for such applications.
One technique [9] for improving the anisotropy of MOSFET detector response places the MOSFET die on the surface of a Kapton tail with the rear of the die and wire bonded to the top side (essentially as described above), but with a dummy Si die of the same thickness as the MOSFET die adjacent to the surface of the MOSFET die so as to overlay an active region of the MOSFET surface gate and surrounding area; an epoxy envelope is again used to provide mechanical fixation and protection from the environment. Sandwiching the sensitive dosimetric micron surface layer (i.e. the MOSFET gate or diode p-n junction) between two Si bulk layers makes the radiation path more isotropic. A similar approach [10] has also been suggested for edge-on MOSFET detectors for the measuring the dose deposited by narrow micron synchrotron X-ray micro-beams in Microbeam Radiation Therapy (MRT), to make scattering conditions uniform when scanning the edge-on MOSFET across the microbeam. However, this approach does not solve the problems of skin dosimetry with the MOSFET detector or diode, increasing the WED of dose measurements and leaving the problem of large WED and poor WED reproducibility due to the epoxy.
Accurate measurement of the skin dose is important in X-ray MV therapy and radiation diagnosis owing to radiation damage of basal layer of epidermis at the depth of 70 to 200 microns. It can be important to determine skin dose due to electron contamination from photon interaction in the air column between the accelerator and patient. The skin dose depends on the incident angle of the beam on the patient, the curvature of the surface of the patient; it increases with increasing beam angle of incidence and beam size, especially with tangential radiation beams in breast cancer treatment. Owing to the absence of a charged particle equilibrium on a surface of the body, the dose gradient is steep which leads to error in skin dosimetry if the WED of the MOSFET measurements is high and a loss of irreproducible within a batch of MOSFETs. Skin dose control in real time is important for avoiding radiation burns, which can lead to serious complications especially during the treatment of breast cancer.
A MOSFET has been reported [11, 14] having a round epoxy envelope as described above and a WED of 1.8 mm for measurements with a 6 MeV X-ray beam with a field size of 10×10 cm. Manual attempts to partially remove the epoxy led to a spreading of the WED in a range 0.04 to 0.15 mm in the same batch of MOSFETs [12], leading to an unacceptable lack of WED reproducibility for skin dosimetry.
Improved anisotropy of the response of the MOSFET with packaging of the type described in reference [8] nonetheless does not provide correct skin dosimetry in radiation therapy.
It has been demonstrated [13] that using a bare (unpackaged) MOSFET allows accurate measurements of dose on the surface on a phantom; this exploits the advantages of the thin dosimetric layer (viz. gate oxide) but it is impractical to leave the gate of the MOSFET unprotected against moisture and mechanical damage.
Temperature stabilization of the sensor response is commonly achieved in the background art by using dual MOSFET sensors and dual bias supplies for differentially biasing the gates of the sensors during irradiation. This technique is employed in [5].
Alternatively, a thermo-stable point on the current-voltage characteristic of the MOSFET may be identified maintained during measurements. However, the readout current corresponding to the thermo-stable point of the current-voltage characteristic is unique for any particular MOSFET and can vary from one batch to another [14].